1. Field of the Invention
The present invention relates to a method for preventing leaking of molten metal in an injection apparatus for metal molding wherein a metal material is supplied as molten metal from a metal material melting and holding furnace into an injection cylinder with a plunger installed therein and both metering and injection of the molten metal are performed by the plunger.
2. Description of the Related Art
When a nozzle installed at a tip of an injection cylinder is touched to a cooled mold to inject molten metal from the injection cylinder into the mold and nozzle touching is continued also after the injection, the nozzle is cooled by coldness transmitted from the mold and the molten metal remaining within the nozzle solidifies to form a metal plug referred to as cold plug. The interior of the nozzle tip is closed with the cold plug and therefore even if the nozzle is separated from the mold, there will not occur any leak of molten metal from the nozzle tip insofar as the separation of the nozzle is after formation of the cold plug (refer to Japanese Patent Publication No. 2001-79653).
On the other hand, if the cold plug is formed to excess, it would be difficult to remove the cold plug even if an injection pressure is applied thereto, thus inducing a large injection resistance. To avoid such excessive formation of the cold plug, the nozzle temperature is controlled or the nozzle tip temperature and the plug forming temperature are calculated in a comparative manner and the nozzle travels backward when the calculated value is within an allowable range (refer to Japanese Patent Publication No. 2004-122141). There also has been proposed a technique of heating the nozzle at the time of injection to soften the environs of the cold plug, thereby permitting the cold plug to be removed more easily by the injection pressure (refer to Japanese Patent Publication No. 2004-122141).
As means for preventing backflow and leak of molten metal supplied into an injection cylinder there have been proposed means wherein a check valve assembly is installed in a molding material communication conduit which is for the supply of molten metal to an injection cylinder (refer to Japanese Patent Publication No. 2004-322200) and means wherein an intermediate shaft portion of a plunger is formed in a valve for opening and closing a supply opening and it closes the supply opening during travel of the plunger up to its most forward position, thereby preventing backflow to a supply path under the action of a dwell force (refer to Japanese Patent Publication No. 2007-38233).